Accurate measurements of temperature and water vapor in the upper-air are of
great interest in relation to weather prediction and climate change. Those
measurements are mostly conducted using radiosondes equipped with a variety
of sensors that are flown by a balloon up to lower stratosphere. Reference
Upper Air Network (GRUAN) has identified water vapor pressure as one of the
most important measurands and has set an accuracy requirement of 2 % in
terms of the mixing ratio. In order to achieve the requirement, many errors
in the humidity measurement such as a temperature dependency in sensing
characteristics including measurement values and response time need to be
corrected because humidity sensors of radiosondes pass through low-pressure
(1 kPa) and low-temperature (−80 ∘C) environments in the
upper-air. In this paper, the humidity sensing characteristics of Jinyang
radiosonde sensors in relation to temperature dependencies were evaluated at
low temperature using a newly developed ultralow-temperature humidity
chamber. The sensitivity characteristic curve of the radiosonde sensors was
evaluated down to −80 ∘C, and the calibration curves of the
humidity sensor and the temperature sensor were obtained. The response time
of humidity sensor slowly increased from 52 to 116 s at the temperature from
20 to −40 ∘C, respectively, and then rapidly increased to almost
one hour at −80 ∘C. Those results will help to improve the
reliability of the upper-air observation data.

The measurement of upper-air temperature and humidity plays an important role
in various fields, e.g., addressing global warming, forecasting weather, and
ensuring aviation safety. In general, the measurement is performed by
launching a radiosonde equipped with temperature and humidity sensors, up to
an altitude of 30 km, to gather and send data back to the ground. However,
upper-air measurements tend to have much poorer reliability than ground-based
measurements due to the extreme environmental conditions, e.g., low air
temperatures down to −80 ∘C or less, low pressure (1 kPa), and
high solar radiation. The 8th international comparison of radiosonde (Nash et
al., 2011) hosted by WMO reported that the measurement discrepancy among
temperature sensors was up to 1.7 ∘C, and the discrepancy was even
larger for humidity sensors, up to 30 % RH. Given that the global
warming trend has been equivalent to about a 1 ∘C rise in the past
century, the accuracy levels of the current upper-air temperature measurement
systems are insufficient to effectively address global warming issues. The
reason for the inaccuracy is that, with regard to temperature measurement,
the heating effect of solar radiation is not properly compensated, and, for
humidity measurement, the sensing characteristics of humidity sensors are
temperature dependent. Therefore, it is necessary to evaluate radiosonde
sensors using an ultralow-temperature humidity chamber (UTHC) which enables
more reliable measurements as well as the traceability of measurements to the
upper air conditions. This effort to develop a measurement
traceability-embedded UTHC and evaluate radiosonde sensors is in line with
the current trend of improving reliability in meteorological measurements.
The WMO executed a mutual recognition agreement (MRA) with the BIPM (Bureau
International des Poids et Mesures) to ensure measurement traceability to the
International System of Units (SI) (WMO-BIPM, 2010).

Furthermore, according to the measurement standards set by the WMO's
Reference Upper Air Network (GRUAN), the uncertainty of measurement is
specified not only for temperature and humidity measurement but also for wind
speed and pressure measurement. These efforts also show that the global trend
in meteorological measurements is moving towards improving reliability. GRUAN
named humidity as one of the most important upper-air measurement items, and
has mandated measurement uncertainty for it is below 2 % in term of the
water vapor mixing ratio (GCOS, 2013).

In an effort to allow the low-temperature calibration of radiosondes
specialized in upper-air measurement, MIKES in Finland developed a UTHC
(Sairanen et al., 2015) that met the uncertainty requirement of the GRUAN
which is 2 % in terms of the mixing ratio. Its UTHC was based on a hybrid
humidity generator principle (Mayer et al., 2008), in which the carrier gas
passing through a saturator and a zero gas supplier mixed in a controlled way
for the humidity generation. In this regard, it allows a fast humidity change
by controlling the mixing ratio and thus can be useful for studying dynamic
responses of humidity sensors in addition to the calibration at static
states. However, the use of the zero gas supplier contributed the major
uncertainty in the uncertainty of the humidity generation. The KRISS UTHC
operates in a two pressure mode which enables a relatively fast humidity
change. The major uncertainty factor is the uncertainty by the device under
test (DUT) in an effort to validate the humidity generation using an
independent hygrometer. A more detailed description on UTHC is being prepared
as an independent paper.

Figure 1Assembled view and photo of main saturator and test chambers with
radiosonde.

A high precision humidity generator, used as reference standard for humidity,
adopts a saturator-based method, where air is saturated under a specific
temperature and pressure, and is subsequently transferred to an environment
of different temperature and pressure to generate the desired humid air.
These methods include a two-temperature method, a two-pressure method, and a
combined two-temperature and two-pressure method (Wiederhold, 1997; Choi et
al., 2012). The UTHC developed in the present study adopted the two-pressure
method, and the setup comprises a saturator, a test chamber, and an expansion
valve. Here, humid air is generated by using a frost-point generation
technique, which uses the saturated water vapor pressure on the surface of
the ice in the saturator. Therefore, analytical interpretation of results is
allowed along with reliable measurement because the approach generates
humidity based on the natural phenomenon regarding condensed material and
vapor pressure. The saturator comprises 11 sub-saturators. Each sub-saturator
contains a spiral-grooved saturator pipe coated with ice. When dry air
travels through this ice-coated pipe, thermal equilibrium is achieved between
the ice coating and water vapor, thus giving rise to saturated humid air. The
saturated humid air is sent to the test chamber with the pressure change made
by the expansion valve. Here, the relative humidity of the test chamber can
be changed by adjusting the pressure drop. A test chamber is where radiosonde
sensors are placed, and in the present study the authors developed various
types of test chambers, e.g., a large-scale test chamber to accommodate up to
five module packages of sensors and a small-scale chamber to quickly change
relative humidity. Figure 1 shows the main parts of the assembled humidity
chamber, i.e., the saturator, test chamber, and expansion valve, along with
the photograph of two different test chambers equipped with a radiosonde. The
saturator and the test chamber were immersed in the same liquid bath. The
expansion valve is used to control the pressure of the saturator and thus
control the humidity in the test chamber. The small-scale test chamber was
used for the evaluation of sensors in this paper since it provides faster
humidity generation. The flow rate through the humidity generator was
1 L min−1 and the air speed is not considered for the evaluation of
sensors. A dew-point meter (MBW 373LX) is connected serially to the gas
outlet of the test chamber.

The generation capacity of the frost-point temperature by the UTHC saturator
is −90 to +50 ∘C while the temperature range of the test chamber
is −80 to +50 ∘C. The maximum pressure of the saturator is
10 MPa. Thus, the relative humidity generation capacity of the test chamber
is 1 % RH to 100 % RH in the temperature range from −80 to
+50 ∘C. The uncertainty of relative humidity generated by the UTHC
is around 1 % RH in the temperature range from −50 to 50 ∘C
whereas the uncertainty increases below −50 ∘C, reaching
1.3 % RH at −60 ∘C and 2 % RH at −80 ∘C. The
uncertainty value at low temperature is mostly contributed by the effect of
adsorption/desorption of moisture on the surface of chamber and tubing line.
It could be evaluated by the difference between the frost-point temperature
generated by the UTHC and that measured by the chilled-mirror hygrometer.

3.1 Temperature dependence of humidity sensitivity response

Radiosonde sensor characteristics were evaluated with the UTHC. The sensors
tested in the present study were provided by Jinyang Industrial Co., Ltd.,
who manufactured the radiosonde. The evaluation of humidity sensors was
carried out as follows: the humidity sensors were first calibrated at room
temperature (20 ∘C); the temperature of the test chamber was
decreased to −80 ∘C; and the temperature was increased step by
step, i.e., -80→-70→-60→-40→-20→-1∘C. The
relative humidity was changed for the sensor evaluation at each temperature.
In order to change the relative humidity in the test chamber, the pressure of
the saturator is varied from 800 kPa for low relative humidity to
atmospheric pressure (∼100 kPa) for high relative humidity while that
of the test chamber is fixed at atmospheric pressure. The temperature and the
pressure of the saturator and the test chamber were measured by platinum
resistance temperature (PRT) sensors and high precision pressure gauges
(Proscientific 745), respectively. Both PRTs and pressure gauges are
calibrated at KRISS. Figure 2a shows the output values of the analogue to
digital converter (ADC) on radiosondes with respect to the capacitance of the
thin-film humidity sensor measured during the calibration at room-temperature
(20 ∘C). Figure 2b shows the conversion of the ADC values to
humidity values by the calibration curve obtained by the linear fitting of
the ADC values. The ADC values during the calibration at room temperature
generally well agree with the linear calibration curve at a mid-range in
relative humidity while they slightly deviate from the calibration curve at
low (below 10 % RH) and high (above 90 % RH) relative humidity.

Figure 2(a) ADC values with respect to the capacitance of
radiosonde humidity sensors as a function of relative humidity and
(b) the conversion of ADC values to relative humidity by radiosonde
humidity sensors (Red and Pink curves) and the reference relative humidity by
the generator (Blue curve) and the hygrometer (green curve) at
20 ∘C.

Figures 3 and 4 show results obtained at each investigated air temperature.
Those are results of the humidity measurement by two radiosonde sensors (Red
and Pink curves) as well as the reference humidity by the generator (Blue
curve) and the hygrometer measurement (Green curve) obtained at each
investigated air temperature. The reference standard can be either the values
by the UTHC or hygrometer. These two values showed essentially no difference
from 20 to −60 ∘C while the hygrometer value started to deviate
from that of UTHC at lower temperature than −60 ∘C. Although these
two references may be united at the equilibrium state after a longer
observation than several hours, the current difference between two values at
−70 and −80 ∘C is about 1.5 % RH. This is because the
equilibrium for the adsorption/desorption of water molecules takes a longer
time as the temperature is lowered, the surface area of the test chamber is
bigger, and/or more humidity sensors are installed. In this regard, the value
of the calibrated hygrometer rather than that of the generator is chosen as
the reference humidity standard in this work. Down to the temperature of
−20 ∘C, the humidity measurements showed no significant deviation
from the reference standard for humidity, as in the case of the
room-temperature measurement. At −40 ∘C, however, the measurements
started to deviate from the standard, especially in the high-humidity ranges.
The deviation increased as the temperature decreased, as shown by the
measurement plot increasingly deviating from the standard plot. This implies
that the sensitivity of the humidity sensors largely depend on the
temperature.

Figure 3Humidity measurements by radiosonde humidity sensors and the
reference humidity by the generator and the hygrometer at −1, −20, and
−40 ∘C.

Figure 5(a) The averaged measurement of relative humidity by two
radiosonde humidity sensors and (b) the corresponding difference
between the measurement and the reference humidity as a function of the
reference humidity at varied temperature.

Figure 6Calculated correction curves for (a) temperature and
(b) humidity sensors as a function of the reference temperature. The
data in (a) is the average of measurements by four radiosonde
temperature sensors and data in (b) is the average of the
measurements by two radiosonde humidity sensors.

Figure 5a shows the humidity measurements by radiosonde humidity sensors as a
function of the reference humidity at each temperature. The corresponding
deviation from the reference humidity is shown in Fig. 5b. From room
temperature down to −40 ∘C, the deviation was within 2 % RH,
but it increased to 10 % RH at −60 ∘C and further to
15 % RH at −80 ∘C. Where, the measurement uncertainties were
less than 1 % RH above temperature −40 ∘C and within
2 % RH at −80 ∘C. During this evaluation of the radiosonde
humidity sensors, the radiosonde temperature sensors, that are thermistors,
are also calibrated through the comparison with the calibrated PRT in the
test chamber at each temperature. Although the data for temperature sensors
is not shown here, the calibration result is shown in Fig. 6a. The
temperature calibration curves of the temperature and humidity sensors were
obtained based on these measurements, as shown in Fig. 6. The calibration
curve of the temperature sensors was expressed as T′=0.0086⋅T+0.2242 in Fig. 6a, while the curve was RH′=0.27⋅T+12.62 (at
20 % RH) and RH′=0.37⋅T+14.15 (at 40 % RH) for the
humidity sensors at −40 ∘C or lower in Fig. 6b. Where, the
uncertainties were not considered because the number of sensors was small.
The results in Figs. 5 and 6 are obtained by increasing the relative
humidity. Although a hysteresis in the humidity measurement by sensors was
observed when increasing and decreasing relative humidity, the hysteresis is
not considered in Figs. 5 and 6.

Figure 7Response time of humidity sensors on radiosonde with decreasing
(down, a) and increasing (up, b) humidity between
20 % RH and 80 % RH at various temperatures.

3.2 Temperature dependence of response time

Since the actual radiosonde measures humidity with flying, at an average
speed of 5 m s−1, the information of response time of sensor is very
important for the correction of the measured values. Figure 7 shows the
measured response time of the humidity sensors of the radiosonde with respect
to temperature. The humidity was varied between 20 % RH and 80 % RH
with increasing and decreasing. Here, the humidity refers to the relative
humidity as defined by the general definition using the saturation vapor
pressure of water over the current phase of water (i.e. water or ice), not by
the WMO definition using saturation vapor pressure of only water including
supercooled water. In case of decreasing humidity, the response time was
about 52 s at room temperature, and as the temperature decreased, it slowly
increased from 53 s at −1 ∘C to 73 s at −20 ∘C. The
response time sharply increased, starting from −40 ∘C at 116 s,
to 294 s at −50 ∘C, to 452 s at −60 ∘C, to 1207 s at
−70 ∘C and further to 3150 s at −80 ∘C. Additionally,
a similar temperature dependency was observed during increasing humidity as
shown in Fig. 7b. Note that the typical response time by the test chamber
itself due to the adsorption/desorption of water molecules was faster than
the response time of the humidity sensor at low temperatures.

The sensitivity characteristic and response time of radiosonde humidity
sensors were investigated at low temperature, using developed
ultralow-temperature humidity chamber which is based on two-pressure humidity
generator. The sensitivity of the radiosonde sensors was measured up to
−80 ∘C, and it was confirmed that the temperature dependency of
humidity characteristics was more than 15 % RH. Based on this,
calibration curves of the humidity sensor and the temperature sensor were
obtained. The response time of humidity sensor from room temperature to
−40 ∘C slowly increased from 52 to 116 s, and then rapidly
increased to almost one hour at −80 ∘C. Since the practical
radiosonde is measured while flying at an average speed of 5 m s−1,
the information on the response time is very important information for the
correction of the measurement data. Evaluation of the radiosonde sensors on
the ground by ultralow-temperature humidity chambers will improve the
accuracy and reliability of the upper-air observation data.

BIC designed the humidity generator, conducted experiments,
and wrote the manuscript. SWL analysed the data and wrote the manuscript. SBW
and JCK constructed the humidity generator and conducted experiment. YGK
designed experiments. SGY interfaced the humidity sensor with the humidity
generator.

This article is part of the special issue “17th EMS Annual
Meeting: European Conference for Applied Meteorology and Climatology 2017”.
It is a result of the EMS Annual Meeting: European Conference for Applied
Meteorology and Climatology 2017, Dublin, Ireland, 4–8 September 2017.

This work was supported by the Korea Research Institute of Standards and
Science under the project “Development of Measurement Standards Technology
as National Infrastructure in Response to the Climate
Change”.

Accurate measurements of temperature and water vapor in the upper-air are of great interest in relation to weather prediction and climate change. Those measurements are mostly conducted using radiosondes. The sensitivity characteristic and response time of radiosonde humidity sensors were investigated at low temperature, using developed ultralow-temperature humidity chamber. This work will improve the accuracy and reliability of the upper-air observation data.

Accurate measurements of temperature and water vapor in the upper-air are of great interest in...